In water-cooled nuclear reactors, the reactor core in which the fission chain is sustained generally contains a multiplicity of fuel assemblies. These fuel assemblies are mechanically identical and interchangeable, each being designed to maintain its own structural integrity.
A fuel assembly typically contains, among other things, a plurality of longitudinally extending parallel members, some bearing fuel and some not bearing fuel. The parallel members are supported vertically by end fittings. Lateral bracing and spacing of the members are provided by spacer grids which contain a plurality of cellular voids. The spacer grids are disposed across the parallel members so that each parallel member extends through one cellular void per grid. The spaces between adjacent members create flow channels through which is circulated water, flowing from bottom to top.
Most of the parallel members contain fuel and are known as fuel rods. The non-fuel bearing members are hollow and comprise an instrument tube and a plurality of guide tubes. The instrument tube accommodates various monitoring devices. The guide tubes are used to receive control rods which regulate the fissioning activity, and hence the generation of heat. Henceforth unless otherwise indicated, the term guide tube will be used to denote instrument tubes as well as guide tubes.
The guide tubes have a larger diameter than the fuel rods. Because of this difference in diameter and because all the cellular voids of the spacer grid have equal center-to-center distances, the flow channel between a guide tube and an adjacent fuel rod has a smaller cross-sectional area than the flow channel between two adjacent fuel rods. This difference in flow channel space affects the flow of water, there being more flow in the wider space between two adjacent fuel rods.
Water flow, in turn, affects the rate of heat transfer from the fuel to the water, a greater flow providing for a greater heat transfer. Since heat transfer rate is a factor affecting the maximum operating conditions (and hence the energy producing capability) of the reactor, the flow of water in the flow channel between a guide tube and an adjacent fuel rod is a limiting factor in the operation of the reactor.